The present disclosure relates to a method and a device for the liquefaction of the CO2 contained in the flue gases.
Most cryogenic methods for the production of CO2 out of combustion flue gases use conventional separation schemes having two or more separation stages. In FIG. 1 such an installation is shown as block diagram.
In the FIGS. 1 and 2 the temperature and the pressure at various points of the flue gas stream as well as of the CO2 are indicated by so-called flags. The temperatures and the pressures belonging to each flag are compiled in a chart in the following. It is obvious for a man skilled in the art that these temperatures and pressures are meant as an example. They can vary depending on the composition of the flue gas, the ambient temperature and the requested purity of the liquid CO2.
In a first compressor 1 the flue gas is compressed. This compression can be a multi-stage compression process with coolers and water separators between each compression stage (not shown) separating most of the water vapour resp. water from the flue gas.
In FIG. 1 the flue gas stream is designated with reference numeral 3. When being emitted by the first compressor 1 the flue gas has a temperature significantly higher than the ambient temperature and then is cooled to approximately 13° C. by a first cooler 5. The pressure is approximately 35.7 bar.
The moisture still contained in the flue gas stream 3 is freed from water by a suitable drying process e.g. adsorption dried in a drier 7 and subsequently conveyed to a first separation stage 9. This first separation stage 9 comprises a first heat exchanger 11 and a first separation drum 13. The first heat exchanger 11 serves for cooling the flue gas stream 3. As a result of this cooling a partial condensation of the CO2 contained in the flue gas stream 3 takes place. Consequently, the flue gas stream 3 enters the first separation drum 13 as a two-phase mixture. There the liquid phase and the gaseous phase of the flue gas stream are separated by means of gravitation. In the first separation drum the pressure is approximately 34,7 bar and the temperature is −19° C. (cf. flag no. 5).
At the bottom of the first separation drum 13 liquid CO2 is extracted and via a first pressure reducing valve 15.1 expanded to a pressure of approximately 18.4 bar (cf. ref. No. 3.1). This results in a temperature of the CO2 between −22° C. and −29° C. (cf. flag no. 10). The partial CO2 stream 3.1 of the flue gases is heated and evaporated in the first heat exchanger 11 by the flue gas stream 3. At the exit of the first heat exchanger 11 the partial stream 3.1 has a temperature of approximately 25° C. and a pressure of approximately 18 bar (cf. flag no. 11).
When the second partial stream 3.2 being extracted at the head of the first separation drum 13 is followed it becomes clear that this partial stream 3.2 being extracted from the first separation drum 13 in a gaseous state is cooled in a second heat exchanger 17 and partially condensed. Afterwards this partial stream 3.2 being also present as two-phase mixture is conveyed to a second separation drum 19. The second heat exchanger 17 and the second separation drum 19 are the main components of the second separation stage 21.
In the second separation drum 19 again a gravity-supported separation between the liquid phase and the gaseous phase of the partial stream 3.2 takes place. In the second separation drum 19 there is a pressure of approximately 34,3 bar and a temperature of approximately
−50° C. (cf. Flag no. 11).
The gaseous phase in the second separation drum 19, the so-called offgas 23, is extracted at the head of the second separation drum 19, expanded to approximately 17 bar in a second pressure reducing valve 15.2, so that it cools down to approximately −54° C.
In the figures the offgas is designated with reference numeral 23. The offgas 23 streams through the second heat exchanger 17 thereby cooling the flue gas 3.2 in the counter stream.
At the bottom of the second separation drum 19 liquid CO2 is extracted and expanded to approximately 17 bar in a third pressure reducing valve 15.3, so that it reaches a temperature of −54° C. as well (cf. flag no. 7a). This partial stream 3.3 as well is conveyed to the second heat exchanger 17. Wherein a part of the liquid CO2 evaporates and a partial stream 3.3.1 is extracted from the second heat exchanger 17, expanded to approximately 5 to 10 bar in a fourth pressure reducing valve 15.4, so that here as well a temperature of −54° C. is reached (cf. flag no. 7b), and again conveyed to the second heat exchanger 17.
After the partial stream 3.3.1 streamed through the second heat exchanger 17, it again is brought together with the partial stream 3.3 and conveyed to the first heat exchanger 11. At the entrance of the first heat exchanger 11 this partial stream has a pressure of approximately 5 to 10 bar with a temperature of −22 to −29° C. (cf. flag no. 14).
This partial stream 3.3 takes up heat in the first heat exchanger 11, so that at the exit of same it has a temperature of approximately −7° C. with a pressure of approximately 5 to 10 bar. The third partial stream 3.3 is conveyed to a second compressor 25 at the first compressor stage, whereas the partial stream 3.1 having a pressure of approximately 18 bar is conveyed to the second compressor stage at the three-stage compressor 25 shown in FIG. 1.
Intercooler between the various stages of the second compressor 25 and an aftercooler for the compressed CO2 are not shown in FIG. 1.
At the exit of the second compressor 25 the compressed CO2 has a pressure of between 60 bar and 110 bar with temperatures of 80° C. to 130° C. In the aftercooler, which is not shown, the CO2 is cooled down to ambient temperature.
If necessary the CO2 can be either fed directly into the pipeline or liquefied and conveyed from a first CO2 pump 27 e.g. into a pipeline (not shown). The first CO2 pump 27 raises the pressure of the liquid CO2 to the pressure given in the pipeline.
Going back to the offgas 23 it can be seen that the offgas streams through the second heat exchanger 17 and the first heat exchanger 11, thereby taking up heat from the flue gas stream 3. At the exit of the first heat exchanger 11 the offgas has a temperature of approximately 26° C. to 30° C. with a pressure of approximately 26 bar (cf. flag no. 16).
For maximising the energy recovery it is known to overheat the offgas 23 with an offgas superheater 29 and then convey it to a expansion turbine 31 or any other expansion machine. Wherein mechanical energy is recycled and afterwards the offgas is emitted into the surroundings with a low pressure approximately corresponding to the surrounding pressure.
This installation described by means of FIG. 1 for liquefying CO2 is relatively simple and works without problems. The disadvantage of this method and this installation for the production of liquid CO2 out of flue gas of power plants e.g. fuelled with fossils is its high energy demand having negative effects on the net efficiency degree of the power plant.